A review of x-ray spectral evolution in Crab pulsar

Pravdo et al (1997) claimed that the phase resolved x-ray spectrum in Crab pulsar (PSR B0531+21) shows a spectral hardening at the leading edge of the first peak of its integrated profile; this was a new and unexpected result. This article reanalyzes their data, as well as some other related data, and argues that the spectrum is as likely to be unvarying (i.e., neither hardening nor softening).Comment: 12 pages, 4 figures, accepted for publication in A&

RXTE observations of single pulses of PSR B0531+21: II Test for radio behavior

This article is the second in the series that analyze about 1.87 million periods of Crab pulsar, observed by the PCA detector aboard the RXTE x-ray observatory. At these energies the pulsar displays none of the three phenomena that are often seen in normal radio pulsars -- ``pulse nulling'', ``systematic sub pulse drifting'' and ``mode changing''. Presence or absence of these three behavior in the Crab pulsar, at radio wavelengths, something that has not been rigorously established yet, might be important for a satisfactory understanding of the above three phenomena.Comment: 6 pages, 4 figure

A non-glitch speed-up event in the Crab Pulsar

Context: The rotation history of the Crab Pulsar is well described by (1) a rotation frequency $\nu$ and a slowdown model that is specified by its first two time derivatives $\dot \nu$ and $\ddot \nu$, known as the secular slowdown model, (2) occasional (once in $\approx 2$ years) significant and abrupt increases in the magnitude of $\nu$ and $\dot \nu$ (occurring on timescales of minutes), known as glitches, and (3) much slower increases and decreases in $\nu$ and $\dot \nu$ (occurring over months and years) that are an order of magnitude smaller, known as timing noise. Aims: This work reports a speed-up event in the Crab Pulsar that occurred around $2015$ February that is distinct from glitches and timing noise. Methods: Monthly $\nu$s and $\dot \nu$s of the Crab Pulsar, obtained at radio frequencies and published by Jodrell Bank Observatory (JBO), are used to demonstrate the speed-up event. Monthly arrival times of the Crab Pulsar's pulse, also published by JBO, combined with X-ray data from the RXTE, SWIFT, and NUSTAR observatories are used to verify the result. Results: The speed-up event is caused by a persistent increase in $\dot \nu$, which results in a monotonic increase in $\nu$. Over the last $\approx 550$ days, $\nu$ has increased monotonically by an amount that is $\approx 10$ times larger than the timing noise level. Conclusions: This is a unique event in the Crab Pulsar. This is probably due to a small increase in the Crab Pulsar's internal temperature. In its absence, the next large glitch in the Crab Pulsar is expected to occur around $2019$ March. However, this event could have an important bearing on its occurrence.Comment: Accepted for publication as a Letter in Astronomy and Astrophysic

RXTE observations of single pulses of PSR B0531+21 I: Flux variations

This article is the first in the series that analyze about 1.87 million periods of PSR B0531+21 (Crab pulsar), observed by the PCA detector aboard the RXTE x-ray observatory. The Crab pulsar's x-ray light curve shows little variation over time scales ranging from days to a period (33.46 milli seconds). The standard deviation of its x-ray flux variation is $\approx$ 0.7% of its mean value, which is negligible compared to its radio flux variations. The phase resolved power spectrum of pulse to pulse x-ray flux variation shows no spectral feature; an upper limit to the peak of any possible broad spectral feature is 0.06% of the mean power. The x-ray fluxes in the two components of its integrated profile are unrelated to each other; their linear correlation coefficient is 0.0004$\pm$0.0010. ``Giant pulses'' that are routinely seen at radio wavelengths are absent here. This work sets very strong constraints on the connection (if any) between the flux variations at radio and x-ray energies, for example due to variation in the degree of coherence of the basic emitters. Its phase resolved x-ray flux variation shows a weak correlation with the integrated profile. If confirmed, this might be an important clue to understanding the x-ray emission mechanism of Crab pulsar.Comment: 6 pages, 4 figures, accepted by Astronomy and Astrophysic